Steel making flux

ABSTRACT

Phosphorus furnace slag is used as a ladle flux and/or as a tundish flux in the continuous casting of steel. The phosphorus furnace slag uniquely combines the properties of thermal insulation, oxidation protection and inclusion absorption necessary to function satisfactorily as a ladle flux and/or as a tundish flux.

FIELD OF INVENTION

The present invention relates to steel making and more particularly tofluxes for the tundish and the ladle of a continuous steel castingprocedure.

BACKGROUND TO THE INVENTION

In the continuous casting of steel, molten steel is poured from a ladleinto a tundish from whence the molten steel passes to the continuouscasting mold. There is an increasing demand from customers of continuouscasters for improved steel cleanliness and more attention is being paidby steel companies to the factors which influence the quantity ofnon-metallic inclusions in steel.

Deoxidation of the steel usually is effected in the ladle and somedeoxidation products are removed by way of ladle slag. However,reoxidation of the steel and dissolved alloys may occur in the ladle,tundish and the continuous casting mold or during one of the transferoperations, thus adversely affecting the final cleanliness in the steel.

There exists, therefore, a need for a flux for both the ladle and thetundish. An ideal ladle flux material and an ideal tundish flux materialshould have the following functions:

(a) Thermal insulation of the steel so as to avoid premature freezingand excessive skulling within the ladle or the tundish,

(b) Protection of the steel from atmospheric oxidation, and

(c) Absorption of inclusions reaching the flux-metal interface.

As is discussed in more detail below, these functions are also requiredof mold fluxes used in the continuous casting mold but there are othermore important properties also demanded of mold fluxes which are notrequired of a ladle flux or a tundish flux. The better the ladle fluxand the tundish flux perform in all three respects, the lesser is theburden imposed on the mold flux in the subsequent continuous castingprocedure and hence the better the mold flux is able to function.

The first function of the ladle flux and the tundish flux requires anouter solid particulate layer since heat transfer through a completelyliquid layer is quite rapid, while the other two functions require aliquid layer in contact with the molten steel. Relatively littleattention has been given to the third requirement but as ladle designsand tundish designs are improved and flow control devices are employedin the ladle and tundish, the extent of inclusion separation is expectedto increase, especially if a suitable flux composition can be providedwhich will absorb the non-metallic reaction products.

It is becoming increasingly standard practice to employ flux material ineither the ladle or the tundish or both. Calcium aluminate is commonlyused as a flux material in the ladle. Rice hulls usually are used as aflux material in the tundish. Rice hulls, which contain approximately85% silica, provide very good thermal insulation and are employed mainlyfor that reason, but are not very effective for oxidation protection orinclusion absorption. Rice hulls are solid at steel-castingtemperatures, so that any extent to which the material fulfils thesecond and third requirements results from fluxing of silica by otheroxides, probably iron and manganese oxides produced by reoxidation, toform a liquid layer, a somewhat unsatisfactory condition.

Various mold fluxes also have been used as tundish fluxes. Thesematerials generally are not very effective in terms of thermalinsulation, unless a powder layer is maintained, although they usuallyprovide reasonably effective inclusion absorption and protection fromoxidation. These materials are formulated to function specifically asmold fluxes and therein lies their drawback as potential tundish fluxes.In addition to the three properties mentioned above, mold fluxes alsoare required to infiltrate between the solidifying steel and the moldwall in the mold. This characteristic, which is the most important onefor a mold flux, much more so than the three functions mentioned above,assists in controlling the rate of heat transfer from the solidifyingsteel to the mold wall and also results in a lubricant for thesolidifying steel as it passes through the mold. In order to provide theflow property, the viscosity of the mold flux at steel-castingtemperatures is generally low, usually as a result of the presence offluidizers, such as sodium oxide, sodium fluoride or calcium fluoride.Carbon also usually is added to control the rate of melting of the moldflux. As a result, the liquidus temperature of mold fluxes is generallyin the range of about 1000° to about 1150° C.

Both the low viscosity and the presence of fluidizers are detrimental inboth the ladle and the tundish environments, however, since, forexample, vortexing in the ladle or tundish as the liquid steel passes tothe continuous casting mold can readily result in gross entrainment ofthe low viscosity material and the fluidizers can cause serious erosionof the refractory material generally used in tundish parts, such as thelining, weirs and ceramic shrouds where submerged nozzles are in use. Inaddition, as a result of their complex formulation, mold fluxes tend tobe rather expensive.

Neither rice hulls nor mold fluxes, therefore, perform satisfactorily inthe ladle or tundish since neither possess all the properties necessaryto function ideally without introducing additional problems. Similarly,calcium aluminate can be unsatisfactory as a flux due to its limitedcapacity to absorb alumina and its ability to transfer hydrogen frommoist air to molten steel. This effect is undesirable and any tundish orladle flux should not enhance the transfer of undesirable elements, suchas hydrogen from atmospheric moisture, as well as from the flux materialitself to the molten steel.

The optimum practical composition for a ladle flux and a tundish fluxhas a somewhat narrow range of properties, as will be seen from thefollowing discussion. Since a liquid layer is desired in contact withthe molten steel, which typically has a temperature in the range ofabout 1475° to about 1600° C. in both the ladle and the tundish, theflux should be completely molten around 1,450° C., but since too low aviscosity causes problems, as discussed above, the superheat should notbe large, so that a liquidus temperature of between 1,350° C. and 1,450°C. is appropriate, a temperature well above that for most mold fluxes.Significant quantities of iron oxides, in the form of FeO or Fe₂ O₃, ormanganese oxide are undesirable since they introduce oxygen into thesteel and may ultimately lead to non-metallic inclusions in thesolidified product. Further, since one of the main functions of the fluxis to absorb inclusions and since most continuously-cast steel slabs arealuminum killed so that the inclusions are alumina or calciumaluminates, the initial alumina content of the flux should be as low aspossible, so as to promote inclusion absorption. As previously observed,the quantities of fluidizers, such as sodium oxide, sodium fluoride andcalcium fluoride, permitted are severely limited by the melting range,viscosity and refractory erosion considerations. The only oxide left tocombine with lime to produce a flux with reasonable meltingcharacteristics is silica.

SUMMARY OF INVENTION

It has now been surprisingly found that phosphorus furnace slagpossesses all the properties necessary for a ladle flux and a tundishflux. In accordance with the present invention, therefore, there isprovided a process for the continuous casting of steel characterized bythe utilization of particulate phosphorus furnace slag as a flux ineither the ladle, the tundish or both.

The phosphorus furnace slag usually is used alone as the flux material,without mixing or compounding with other components. Minor quantities ofother components may be present for special purposes not directlyrelated to the fluxing activity.

The phosphorus furnace slag is particularly effective as a flux in theladle or tundish, in that it meets all three of the criteria notedabove.

Phosphorus furnace slag has a melting point such that a molten layer isformed on the metal surface from the powder slag layer covering themolten metal in the tundish or ladle. The remainder of the coatingremains in powder form, thereby providing thermal insulation of themolten metal and preventing freezing and excessive skulling within theladle or the tundish.

The presence of the liquid layer on the surface of the molten metaleffectively seals off the molten metal from surrounding air, and therebyprotects the steel from atmospheric oxidation. As discussed in moredetail below, the chemical composition of phosphorus furnace slag is lowin alumina, which permits absorption of alumina and similar inclusionsreaching the molten flux-molten metal interface. In addition, phosphorusfurance slag does not enhance the transfer of other undesirableelements, including hydrogen, to the molten steel.

GENERAL DESCRIPTION OF INVENTION

Phosphorus furnace slag is obtained as a low value by-product of theproduction of yellow phosphorus by the electrothermal reaction ofphosphate rock, silica and carbon, and hence is inexpensive. The slag istapped from the arc furnace and usually is allowed to cool gradually. Inthis form, the phosphorus furnace slag has a crystalline structure andmay be employed in that form, after provision in an appropriate particlesize range.

Alternatively, the slag may be provided in an amorphous form for useherein. One manner of formation of the amorphous form of the slag is togranulate molten slag removed from the furnace by quenching in water.Another manner of formation of the amorphous form of the slag is bypelletizing the water quenched molten slag by the ejectory action of awater-cooled rotating drum and collecting the pellets so formed. Theamorphous form of phosphorus furnace slag is preferred for use in thepresent invention, since the chemical composition is substantiallyhomogeneous, whereas, in the case of crystalline slag, variations inchemical composition are common throughout the mass, which may beundesirable.

The exact composition of the phosphorus furnace slag varies, dependingon the proportions of the starting materials employed in the phosphorusfurnace. Generally, the composition of phosphorus furnace slag may beconsidered to be a fluorine-containing calcium silicate having a weightratio of CaO to SiO₂ in the range of about 0.7 to about 1.35, preferablyabout 0.8 to about 1.2, containing less than about 5 wt.% fluorine. Thephosphorus furnace slag also usually contains a variety of othercomponents, examples of which are set forth in Table I below.

Phosphorus furnace slag produced from a typical electrothermal yellowphosphorus production process meets all the above-noted criteria to asignificant degree, indicating that the material may be employed as aladle flux and/or as a tundish flux. The liquidus temperature ofphosphorus furnace slag usually is between about 1300° to about 1500°C., depending on the CaO/SiO₂ ratio of the slag, i.e., within thedesired range discussed above. The liquidus temperature in this rangerelative to a molten steel temperature of about 1475° to about 1600° C.in both the ladle and the tundish, results in a liquid layer of moltenslag on the surface of the metal while the remainder of the powderedslag remains in solid particulate form.

This result has a two-fold effect. The residual particulate materialensures that there is thermal insulation of the body of metal in theladle or tundish while the liquid layer effectively seals offatmospheric oxygen from the molten metal, ensuring that oxidation of themetal does not occur.

The chemical composition noted above for the phosphorus furnace slagalso conforms to that desired for a ladle or a tundish flux, containinglow amounts only of Al₂ O₃, P₂ O₅, Fe₂ O₃ and F. Typical compositionsfor phosphorus furnace slags produced in commercial yellow phosphorusplants produced under differing conditions are set forth in thefollowing Table I:

                  TABLE I                                                         ______________________________________                                                 A.sup.(1) B.sup.(2)   C.sup.(3)                                      Component   % by wt                                                           ______________________________________                                        CaO        45 to 47    48 to 50    48.45                                      SiO.sub.2  43 to 45    40 to 42    43.93                                      (wt ratio  (1.0 to 1.09)                                                                             (1.14 to 1.25)                                                                            (1.10)                                     CaO:SiO.sub.2)                                                                Al.sub.2 O.sub.3                                                                         2 to 3      3           2.92                                       Fe.sub.2 O.sub.3                                                                         0.5         0.1         0.26                                       P.sub.2 O.sub.5                                                                          1.5         0.45        0.7                                        F          3           3-3.5       2.05                                       MgO        0.6         U.sup.(4)   U                                          Na.sub.2 O 0.7         U           U                                          ______________________________________                                         Notes:                                                                        .sup.(1) Typical formulation range for phosphorus furnace slag produced       under a first set of production conditions                                    .sup.(2) Typical formulation range for phosphorus furnace slag produced       under a second set of production conditions                                   .sup.(3) Formulation of amorphous phosphorus furnace slag given in U.S.       Pat. No. 4,340,426                                                            .sup.(4) U means unknown                                                 

In the utilization of phosphorus furnace slag as a ladle flux and/or asa tundish flux, the product is applied to the molten steel in the ladleor the tundish as a particulate layer of a thickness such that theportion of the layer immediately adjacent the steel becomes liquid whilethe outer portion remains in solid form. In this way, the solid portionachieves thermal insulation of the steel surface while the liquidportion protects the steel from atmospheric oxidation and also acts asan absorption medium for inclusions reaching the flux-metal interface.

A broad range of particle size of particulate phosphorus furnace slagmay be employed in the present invention. The range of particle size mayvary from pellets to finely pulverized powder, with the choice dependingon the physical form in which the phosphorus furnace slag is available.

The following Table II provides a screen analysis of two typical samplesof pelletized amorphous slag which may be employed in this invention.

                  TABLE II                                                        ______________________________________                                                       Sample 1                                                                             Sample 2                                                Screen Size       (wt. %)                                                     ______________________________________                                        + 1/2 in.        0        0                                                   - 1/2 in. + 3/8 in.                                                                            2.2      0.7                                                 - 3/8 in. + 1/4 in.                                                                            U        4.4                                                 - 1/4 in. + 4 mesh                                                                             14.1     9.1                                                 - 4 + 6          15.4     19.0                                                - 6 + 8          24.9     27.8                                                - 8 mesh         43.9     37.0                                                ______________________________________                                    

The pellets of amorphous slag may be reduced to a smaller particle size,if desired.

The crystalline form of the phosphorus furnace slag usually is availablein the form of a solidified mass, which is conveniently reduced to fineparticulate form by pulverizing for use herein. However, larger particlesizes of the crystalline form also may be employed.

In some instances it may be desirable to include small quantities ofmagnesia in the flux composition, perhaps as a top covering on the slag,so as to buffer the slag and minimize any tendency to dissolvemagnesia-based refractories used in the ladle and/or the tundish.

The phosphorus furnace slag is sufficiently low in alumina that it isable to absorb significant quantities of alumina, typically about 20 to30 wt.%, before the melting point and viscosity are adversely affected.When the flux becomes exhausted, it may be removed from the steelsurface in the ladle or the tundish in any convenient manner andreplaced by fresh material. For example, removal of the spent flux fromthe tundish conveniently may be effected by raising the level of moltensteel in the tundish to an overflow level at which the spent flux may beskimmed or sloughed off. Raising the molten steel level and restorationto the normal level may be achieved by suitable throttling of the flowof molten steel from the tundish to the continuous casting mold or byincreasing the steel flow from the ladle.

The applicants are aware that it has previously been suggested in U.S.Pat. No. 4,340,426 to use amorphous phosphorus furnace slag in acontinuous casting mold flux composition. As is described in thatpatent, the mold flux composition usually comprises not only thephosphorus furnace slag but also quantities of an alkali agent, such asan alkali silicate glass, and carbon. The purpose of the presence ofthese additional constituents has been discussed above with respect tothe general criteria for mold fluxes. The presence of these additionalmaterials lowers the melting temperature and renders the resultingcomposition undesirable for use as a ladle flux or as a tundish flux.There is no disclosure or suggestion in this prior are patent toindicate that it might be possible to utilize particulate phosphorusfurnace slag alone, or in combination with minor amounts of othermaterials, as a flux in the ladle and/or tundish of a continuous steelcasting process.

The disclosure herein of the utility of the phosphorus furnace slag as aladle and/or tundish flux has been mainly directed to aluminum killedsteels. However, the same material, i.e. phosphorus furnace slag, may beused with steels deoxidized with manganese and silicon to achieve asignificant improvement in the tundish when compared with the use ofrice hulls as a tundish flux.

EXAMPLES Example 1

Samples of commercially-formed powdered amorphous and crystallinephosphorus furnace slag were tested with respect to their viscosity atvarious temperatures, typical of those encountered in a molten steelenvironment in the ladle or tundish of a continuous casting plant.

Two groups of samples having differing CaO/SiO₂ ratios were tested. Forthe first group of samples, the CaO/SiO₂ weight ratio of 1.01 while forthe second group of samples, the CaO/SiO₂ weight ratio was 1.18. Bothgroups of samples had an Al₂ O₃ content of less than 3 wt.%.

The viscosity determinations were made using the same equipment forviscosity measurements of mold powders. The sample is melted andcollected in the Herty viscometer. The Herty viscosity then is thedistance the melt has travelled down the tube (in cm). The Hertyviscosity of slag having a known composition and known dynamic viscosity(in poise (P)) also was determined and used as a calibration curve todetermine the dynamic viscosity of the commercial slag samples.

The viscosity measurements are set forth in the following Table III:

                                      TABLE III                                   __________________________________________________________________________    Viscosity of Phosphorus Furnace Slag                                                      1450° C.                                                                      1500° C.                                                                     1550° C.                                                                      1600° C.                               Sample description                                                                        (cm)                                                                             (P) (cm)                                                                             (P)                                                                              (cm)                                                                             (P) (cm)                                                                             (P)                                        __________________________________________________________________________    (i) Group I Samples:                                                          1. Amorphous                                                                              7.0                                                                              1.3 7.0                                                                              1.3                                                                              6.5                                                                              1.5 7.0                                                                              1.3                                        2. Amorphous pelletized                                                                   8.0                                                                              0.9 7.5                                                                              1.1                                                                              7.5                                                                              1.1 8.0                                                                              0.9                                        3. Crystalline                                                                            5.0                                                                              2.1 5.5                                                                              1.7                                                                              6.5                                                                              1.5 7.0                                                                              1.3                                        (ii) Group II Samples:                                                        4. Amorphous                                                                              6.5                                                                              1.5 8.5                                                                              0.7                                                                              10.0                                                                             0.1 10.0                                                                             0.1                                        5. Amorphous pelletized                                                                   6.0                                                                              1.7 8.0                                                                              0.9                                                                              10.5                                                                             0.1 10.0                                                                             0.1                                        6. Crystalline                                                                            5.5                                                                              1.7 7.5                                                                              1.1                                                                              8.0                                                                              0.9 8.5                                                                              0.7                                        __________________________________________________________________________     Note:                                                                         Samples 2 and 5 were pulverized before testing and therefore were             equivalent to samples 1 and 4, respectively.                             

As may be seen from the above Table III, all the samples were liquid atthe experimental temperature, which corresponded to typical ladle andtundish temperatures, and which indicates that they are effective inpreventing oxidation of molten steel in a ladle or tundish. Theviscosity decreased with increasing temperature and the effect was morepronounced for the first group of samples than the second group. Inaddition, the viscosity of amorphous slag is slightly lower than that ofcrystalline slag.

Example 2

The viscosity of slag samples with varying quantities of alumina wasdetermined as described in Example 1. Samples were prepared by mixingslag with reagent grade alumina and Herty viscosity was determined at1550° C. Dynamic viscosity was determined by calibration, as describedin Example 1. The results are set forth in the following Tables IVA andIVB:

                  TABLE IVA                                                       ______________________________________                                        Herty Viscosity of Phosporus Furnace Slag                                     With Added Al.sub.2 O.sub.3 (cm)                                              Sample Description                                                                           5%      10%    15%   20%  25%                                  ______________________________________                                        (i) Group I Samples:                                                          Amorphous      6.3     4.9    4.3   3.7  2.9                                  Crystalline    6.5     5.5    4.5   3.5  3.0                                  (ii) Group II Samples:                                                        Amorphous      5.6     4.9    4.2   2.8  2.7                                  Crystalline    7.0     5.5    5.0   2.5  2.0                                  ______________________________________                                    

                  TABLE IVB                                                       ______________________________________                                        Dynamic Viscosity of Phosphorus Furnace Slag With Added Al.sub.2 O.sub.3      (P)                                                                           Sample Description                                                                           5%      10%    15%   20%  25%                                  ______________________________________                                        (i) Group I Samples:                                                          Amorphous      1.6     2.1    2.4   2.6  3.0                                  Crystalline    1.5     1.7    2.3   2.7  2.9                                  (ii) Group II Samples:                                                        Amorphous      1.9     2.1    2.4   3.0  3.1                                  Crystalline    1.3     1.7    2.1   3.1  3.4                                  ______________________________________                                    

As may be seen in the above Table IV, the viscosity of all slag samplesincreased with increasing alumina concentration. The alumina absorptiondecreased with increasing viscosity but up to 25 wt.% alumina absorptionwas observed. These results show that alumina contained in molten steelmay be removed in significant amounts within the tundish and ladle.

Example 3

The shroud erosion index (SEI) for fused silica by the phosphorusfurnace slag samples was determined. The shroud erosion index is anindication of the relative erosion of a shroud test piece (fused silica)by the test material.

Slag was heated to 1500° C., the fused silica was dipped into theresulting melt for four hours and the resulting erosion was measured.The erosion determined then was compared to that of a similar shroudtest piece dipped into a standard mould powder under the sameconditions. The SEI test was repeated at 1550° and 1575° C. The resultsobtained are set forth in the following Table V:

                  TABLE V                                                         ______________________________________                                        Shroud Erosion Index of Slag Samples*                                         Sample Description                                                                           1500° C.                                                                         1550° C.                                                                         1575° C.                            ______________________________________                                        (i) Group I Samples:                                                          Amorphous      0.97      0.89      0.86                                       Crystalline    0.61      0.60      0.62                                       (ii) Group II Samples:                                                        Amorphous      1.08      1.05      0.98                                       Crystalline    0.92      0.78      0.75                                       ______________________________________                                         *The comparison of shroud erosion index was with respect to the mold          powder known by the trademark SCORIALIT 1/STG, for which the SEI is           considered to be 1.                                                      

As may be seen from the results outlined in Table V, all the slagsamples gave the same or slightly smaller erosion of fused silica testpieces than Scorialit 1/STG, with smaller numbers corresponding to lowererosion. The SEI did not change significantly with increasingtemperature. The crystalline slag was found to be less corrosive thanthe amorphous slag and the Group II samples were slightly worse than theGroup I samples.

Example 4

The effect of phosphorus furnace slag on tundish liner material wasdetermined. Test pieces of "GARNEX" (trademark) DR658-26 tundish linerwere cut in dimensions 1 inch×1 inch×5 inch. An attempt to duplicate theSEI test of Example 3 on this material failed, mainly because theorganic binder present in the tundish liner was burned off under theconditions of testing. The test pieces turned to powder but theresulting material did not dissolve in the molten slag.

The results of the testing set forth in Examples 1 to 4 demonstrate thesuitability of phosphorus furnace slag material, either in amorphous orcrystalline form, as tundish or ladle fluxes in steel making.

SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention provides a novelutility for phosphorus furnace slag as a tundish and/or ladle flux incontinuous steel casting. Modifications are possible within the scope ofthis invention.

What we claim is:
 1. In the casting of steel wherein molten steel islocated in a ladle, a tundish or both, the improvement which comprisesutilizing as a flux in at least one of said ladle and said tundish, anadditive consisting essentially of particulated phosphorus furnace slagin an amount sufficient to cover the surface of the molten steel and toprovide a molten layer thereof in contact with the surface of the moltensteel and a solid powder layer in contact with the molten layer, saidphosphorus furnace slag being provided as a by-product from theproduction of yellow phosphorus by the electrothermal reaction ofphosphate rock, silica and carbon and having a weight ratio of CaO toSiO₂ of about 0.7 to about 1.35.
 2. The process of claim 1 wherein saidphosphorus furnace slag is in crystalline form.
 3. The process of claim1 wherein said phosphorus furnace slag is in amorphous form.
 4. Theprocess of claim 1 wherein said phosphorus furnace slag has a particlesize ranging from pellets to very finely pulverized particles.
 5. Theprocess of claim 1 wherein said phosphorus furnace slag has a weightratio of CaO to SiO₂ of about 0.8 to about 1.2.
 6. The process of claim1 wherein said phosphorus furnace slag has a weight ratio of CaO to SiO₂of about 0.8 to about 1.2 and contains less than 5 wt.% fluorine.
 7. Theprocess of claim 1 wherein said phosphorus furnace slag has a liquidustemperature of about 1300° to about 1500° C.
 8. The process of claim 1wherein said steel is aluminum-killed steel which is cast by acontinuous operation.
 9. The process of claim 1 wherein said steel ismanganese-silicon killed steel which is cast by a continuous operation.